High intensity discharge (HID) lamps produce light by striking an electrical are across electrodes housed inside a fused quartz or fused alumina arc chamber. The chamber encloses specific components such as mercury vapor, metal halide, alkali and rare earth metals which are selected based on the wavelength of the radiant emission of the excited states of the metallic components.
Standard low-pressure sodium lamps have the highest efficiency of all HID lamps, but they produce a yellowish light. High-pressure sodium lamps that produce a whiter light, but efficiency is somewhat sacrificed. Metal halide lamps are less efficient but produce an even whiter, more natural light. High-intensity discharge (HID) lamps, typically require power supplied by either magnetic or electronic ballasts. Magnetic ballasts provide electrical power to the HID lamp during normal steady-state operation typically at power line frequency, e.g. 50-60 Hz and electronic ballasts provide electrical power to the HID lamp typically at a low-frequency, e.g. 120 to 200 Hz square wave, quasi-sine, pure sine wave or rectangular waveform.
High intensity discharge (HID) lamps suffer from acoustic resonances when HID lamps are operated at high frequencies, i.e., between a few kHz to about two hundred kHz, depending on the dimensions of the lamp. Acoustic resonance causes the radiant arc within the lamp to gyrate, flicker, and even be extinguished. When acoustic resonance occurs and the arc extinguishes typically within milliseconds, hot restart becomes a major limitation in such a lamp-ballast system. However, when the lamps are operated at high frequencies, i.e., above the highest acoustic resonance which depends on the dimensions of the lamp (e.g. ˜50-120 kHz for a 400 Watt metal halide lamp), lamp performance is not adversely affected. Consequently, there are manufacturers of HID electronic ballasts which power the lamp with high-frequency power, at frequencies just beyond the acoustic resonance range. The frequency of high frequency electronic ballasts is conventionally selected to be high enough to avoid acoustic resonances, but not so high as to increase cost and complexity of the ballast circuit.
The issue of acoustic resonance is further compounded by the fact that the acceptable acoustic window is different for arc tubes made even by the same lamp producer and between different lamp manufacturers. As a result, HID lamp interchangeability is a major constraining factor for both new and retrofit installations. Even if the high frequency ballast and HID lamp are compatible when initially installed, characteristics of the arc tube and/or electrodes change over time and could still result in an acoustic resonance condition later in the lamp life cycle.
There is thus a need for, and it would be highly advantageous to have an electronic ballast which ignites and operates HID lamps, avoiding acoustic resonance at high frequency and universally operates different types of lamp, similar lamps of different manufacturers, whiling maintaining performance over time during the lifetime of the HID lamps.
Luminous flux is a quantitative expression of the brilliance of a source of visible light, which is electromagnetic energy within the wavelength range of approximately 390 nanometers (nm) to 770 nm. This quantity is measured in terms of the power emitted per unit solid angle from an isotropic radiator, a theoretical point source that radiates equally in all directions in three-dimensional space.
The standard unit of luminous flux is the lumen (lm). Reduced to base units in the International System of Units (SI), 1 lm is equivalent to 1 candela steradian (cd·sr). This is the same as 1.46 milliwatt of radiant power at a wavelength of 555 nm, which lies in the middle of the visible spectrum. Lux is a derived unit based on lumen, and lumen is a derived unit based on candela. One lux is equal to one lumen per square meter, where 4π lumens is the total luminous flux of a light source of one candela of luminous intensity.